A UV-visible (UW) technique was used to monitor the evolution of transparency during film formation from hard latex particles. Two different latex films were prepared from particles with high and lon molecular weight (HM and LM) poly(methyl methacrylate) (PMMA) separately and annealed at elevated temperatures in various time intervals above the glass transition temperature (T-g). In both films, a continuous increase in the transmitted photon intensity (I-rf) was observed above 160 degrees C as the annealing temperature was increased. However, the reflected photon intensity (I-rf) first decreased and then increased by showing a minimum in the same temperature range as the annealing temperature was increased. The increase in the transmitted photon intensity (I-tr) is attributed to increase in the "crossing density" at the junction surface. The activation energies for back-and-forth motion (Delta E-tr) were measured and found to be around 35 and 25 kcal/mol for the reptating polymer chain across the junction surface in the LM and HM films, respectively. The decrease in I-rf was explained by the void-closure mechanism, and the increase in the I-rf above 160 degrees C was again attributed to the increase in the crossing density at thejunction surface. Back-and-forth activation energies (Delta E-rf) were measured to be around 47 and 18 kcal/mol and the void-closure constants (B) were found to be around 24 x 10(3) and 12 x 10(3) K for the LM and HM film samples, respectively. (C) 1998 John Wiley & Sons, Inc.